1. Technical Field of the Invention
The present invention relates, in general, to the mobile communications field and, particularly, to methods for determining the location of a mobile station within a wireless network.
2. Background and Objects of the Invention
Mobile radio position determination is an emerging field that has gained a great deal of interest lately, and it is now desirable to include a position determination capability in future cellular mobile radio systems. The Time Difference of Arrival (TDOA) position determination method, which is known from military applications, has been used successfully for determining the position of mobile radio terminals. A typical TDOA position determination system can be either terminal based, whereby Time Of Arrival (TOA) measurements are made on the xe2x80x9cdownlinkxe2x80x9d in the mobile station (MS) , or network based, whereby the network performs the TOA measurements on the xe2x80x9cuplinkxe2x80x9d in the radio base transceiver stations (BTSs). These TOA measurements are then used to calculate TDOA parameters and estimate the position of the mobile station (MS).
In order to determine the position of a mobile station, a handover procedure is initiated. The mobile station transmits access request signals to a new base transceiver station. The base transceiver station measures the time delay for the access request signal to travel between the mobile station and the base transceiver station. This procedure is repeated between the mobile station and one or more additional base transceiver stations. A service node in the cellular network calculates the position of the mobile station by utilizing information about the known positions of the base transceiver stations and the measured access time delays.
This network-based method of determining the position of cellular mobile stations relies on so-called asynchronous handovers, where the target base transceiver stations measure the access delays to the mobile station. Each access delay is used as a measure of the distance between the mobile station and the respective base transceiver station. In specific situations, i.e., MS positioning, a number of BSs may concurrently tune to the same transmitting MS in order to make time delay of arrival measurements between the MS and the BSs. The time delay of the signal transmitted by the mobile station during a handover procedure may be measured by the BSs for use by a processing unit to determine the position of the mobile station. The mobile station transmits signals during the handover operation as a series of bursts.
While the MS transmits the access burst, however, the MS is unable to transmit voice signals on a traffic channel. Therefore, an undesirable speech interruption results for the duration of the MS""s internal counter, e.g., for 320 milliseconds. However, access bursts for the entire 320 milliseconds are not always necessary to position the MS with an acceptable degree of precision. For example, a positioning request by an Emergency Service Bureau generally requires a high degree of precision while an application which detects home zone calls requires a lower degree of precision. Nevertheless, because the BTS does not send physical information to the MS, the MS continues to send access bursts during the 320 milliseconds until the internal counter expires even in situations where the target BTS does not require 320 milliseconds of access bursts to achieve the requisite degree of accuracy in measuring the distance between the BTS and the MS. Therefore, the positioning process results in unnecessary speech interruptions. Accordingly, there is a need to minimize undesirable speech interruptions by controlling the number of access bursts in accordance with the requisite degree of accuracy required.
The present invention is directed to a system, method, and apparatus for controlling access bursts from a mobile station during mobile station positioning to a level commensurate with the degree of precision required for the positioning. The mobile station transmits access bursts which are used to determine the distance of the mobile station from a particular point. When the distance of the mobile station from the particular point is determined within a degree of precision, which correlates to the degree of precision required for the positioning, a signal is transmitted to the mobile station which causes the mobile station to cease transmitting the speech-overriding access bursts. By controlling the duration of access burst transmissions to a level commensurate with the degree of precision required for positioning, deleterious speech interruptions during a positioning handover are reduced or eliminated.